The present invention relates to offshore producing platforms and particularly to platforms that are limited as to the amount of currentinduced drag they can withstand and the suspended weight they can support. All floating production platforms that can be installed in deep water have this limitation including moored, tension leg, and guide tower type platforms.
In many offshore producing wells it is desirable to use a dual bore subsea production tree so that controlled access is available to both the production tubing and to the annulus surrounding the production tubing which extends into the well. By having controlled access to both the production tubing and the annulus surrounding the production tubing, it is possible to fill the well with weighted drilling mud to kill the well when it is necessary to remove the production tubing for workovers or other operations in the well. In the absence of separate controlled access to the annulus, the production tubing string must be used to introduce weighted drilling mud into the well to control the well so that the production tubing string could be removed. This is not desirable since the production fluid flowing in the tubing string will be under pressure and thus the operations must be conducted under the pressure. In contrast, if one has direct access to the annulus surrounding the production tubing string, there will be no need to contend with the production fluid flowing in the tubing string while the well is being killed.
While dual bore tubing hangers and dual bore subsea production trees are available, they require the use of separate tubing strings in the two bores. In addition, both of the tubing strings must be enclosed within a marine riser that extends from the wellhead to the surface platform. This requires that the surface platform be capable of supporting the suspended weight of the two tubing strings plus the marine riser.